1. Field of the Invention
The present invention generally relates to CCD (charge coupled device) imaging devices and, more particularly, to a CCD linear sensor.
2. Description of the Prior Art
CCD linear sensors transfer signal charges of odd- and even-numbered sensor elements (pixels) through two charge transfer registers, respectively and output the thus transferred signal charges from an output terminal of the final stage in order to improve a resolution.
FIG. 1 of the accompanying drawings shows an overall structure of a conventional CCD linear sensor.
Referring to FIG. 1, first and second horizontal transfer registers 4 and 5 of CCD structure are disposed through read-out gate sections 2 and 3 on both sides of sensor region 1 in which a plurality of sensor elements (pixels) S (S.sub.1, S.sub.2, S.sub.3, . . . ) are arrayed in one direction, whereby signal charges of the odd-numbered sensor elements S.sub.1, S.sub.3, S.sub.5, . . . are read out to the first horizontal transfer register 4 while signal charges of the even-numbered sensor elements S.sub.2, S.sub.4, S.sub.6, . . . are read out to the second horizontal transfer register 5. Then, the respective signal charges are transferred in one direction by two-phase drive pulses .phi.H.sub.1 and .phi.H.sub.2 shown in FIGS. 2A and 2B. Transfer sections HR.sub.1 and HR.sub.2 of the final stages of the first and second horizontal transfer registers 4 and 5 are connected to a floating diffusion region 7 through a common horizontal output gate section 6 to which there is applied a gate voltage V.sub.HOG. Then, signal charges from the first and second horizontal transfer registers 4 and 5 are alternately transferred to the floating diffusion region 7, whereby they are converted into signal voltages and signals corresponding to the odd-numbered and even-numbered pixels which are alternately output from an output amplifier 8 as shown in a waveform of an output of the CCD. In the output section, the signal charges transferred to the floating diffusion region 7 are sequentially drained out through a reset gate section 9 to a reset drain region 10 by a reset pulse .phi.RG which is higher in frequency than the drive pulses .phi.H.sub.1 and .phi.H.sub.2 as shown in FIG. 2C.
FIG. 3 of the accompanying drawings shows an enlarged plan view of the conventional first and second horizontal transfer registers 4 and 5. The first and second horizontal transfer registers 4 and 5 respectively comprise a plurality of transfer sections HR (HR.sub.1, HR.sub.2) having transfer electrodes 13R (13R.sub.1, 13R.sub.2), each having a set of storage electrodes 12S (12S.sub.1, 12S.sub.2) made of a polycrystalline silicon first layer and transfer electrodes 12T (12T.sub.1, 12T.sub.2) made of a polycrystalline silicon second layer. The drive pulse .phi.H.sub.1 is applied to every other one of the transfer electrodes 13R.sub.1 and the drive pulse .phi.H.sub.2 is applied to remaining every other one of the transfer electrodes 13R.sub.2. When signal charges of the sensor elements S are read out to the first and second horizontal transfer registers 4 and 5, the signal charges are read out to the region (so-called storage section) of the first storage electrode 12S.sub.1 through the region (so-called transfer section) of the first transfer electrode 12T.sub.1 to which the drive pulse .phi.H.sub.1 is applied.
Hence, as shown in FIG. 3, the first transfer electrodes 12T.sub.1 to which the drive pulse .phi.H.sub.1 is applied are shaped as a comb whose respective one ends are coupled continuously. The coupled portions thereof are provided so as to overlap the read-out gate sections 2 and 3. On the other hand, the second transfer electrodes 12T.sub.2 to which the drive pulse .phi.H.sub.2 is applied are disposed between the first transfer electrodes 12T.sub.1, and the corresponding storage electrodes 12S.sub.1 and 12S.sub.2 are disposed between the adjacent transfer electrodes 12T.sub.1 and 12T.sub.2.
In the above-mentioned CCD linear sensor, since the pattern configurations of the first and second transfer electrodes 12T.sub.1 and 12T.sub.2 are different, the first and second transfer electrodes 13R.sub.1 and 13R.sub.2 in the horizontal transfer registers 4 and 5 are different in area. As a result, the capacity of the first transfer section HR.sub.1 to which the drive pulse .phi.H.sub.1 is applied and that of the second transfer section HR.sub.2 to which the drive pulse .phi.H.sub.2 is applied are not equal to each other, thereby a difference is produced between the signal level of the odd-numbered pixel (sensor element) and that of the even-numbered pixel (sensor element).
More specifically, in the output section, the floating diffusion region 7 is formed within the well region, and the well region is interconnected to the ground. In that case, however, the capacities of the transfer sections HR.sub.1 and HR.sub.2 are large so that, when the horizontal transfer registers 4 and 5 are driven by the two-phase drive pulses .phi.H.sub.1 and .phi.H.sub.2, a potential (known as "reference potential") Vo of the well region momentarily fluctuates at the negative-going edges t.sub.1, t.sub.2, t.sub.3, . . . of the drive pulse .phi.H.sub.1 or .phi.H.sub.2 which is used to transfer the signal charge to the floating diffusion region 7, as shown in FIG. 2. As a result, a noise component 16 is mixed into the output signal due to the fluctuation component 15. If the fluctuation component 15 of the potential Vo in the well region are the same, then no problem will occur. However, since the first and second transfer sections HR.sub.1 and HR.sub.2 are different in capacities, the output signal has different noise components 16, thereby causing a difference (so-called DC level difference) to occur between the signal level of the odd-numbered pixel and that of the even-numbered pixel.
This signal level difference becomes conspicuous as the CCD linear sensor is increased in resolution. Hence, this signal level difference must be corrected at every pixel.